Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or pulled through a die to form the desired cross-section. Extrusion may be continuous (indefinite length) or semi-continuous (producing many pieces). The temperature and malleability of the material affects the rate at which the extrusion process occurs, which itself generates heat from work and friction, as well as the type of machinery used to perform the extrusion. Extrusion can be used to create complex cross-sections and to work materials that are brittle, because the material only encounters compressive and shear stresses. Metal is a commonly extruded material.